With a measurement, measurement values in a parameter range of interest of the at least one parameter may be provided for a variable which depends on at least one parameter. Depending on the scope of the parameter range and the accuracy of the measurement, carrying out the measurement in the whole parameter range may be connected with much outlay and with high costs of a measuring device used for the measurement. This may be avoided by virtue of only obtaining measurement values of the variable in subregions of the parameter range. These may be disjoint, i.e. separate and non-overlapping, subregions.
By way of example, such a procedure may be considered in an optical inspection system, which serves to examine an object, e.g. a lithography mask or reticle. The inspection system may comprise an illumination source for illuminating the object arranged in an object plane with radiation and an imaging optical unit. It is possible to image an object field into an image field of an image plane with the imaging optical unit. It is possible to use a sensor arranged in the image plane for radiation measurement purposes.
In the inspection system, it may be of interest to identify an imaging or wavefront aberration in the image field that is caused by the imaging optical unit. Here, the wavefront aberration constitutes the aforementioned variable and the image field constitutes the parameter range, in the present case with two parameters in the form of spatial coordinates. Instead of carrying out the radiation measurement for providing measurement values of the imaging aberration in the entire image field, the measurement may relate to disjoint subregions or partial fields of the image field. By way of example, this may be realized by virtue of a sensor used for the measurement being successively arranged at different measurement positions. In so doing, a plurality of partial measurements are carried out in succession with the sensor and the sensor is moved to the individual measurement positions therebetween. A radiation measurement with a sensor which has partial sensors with non-overlapping detection regions is also possible.
A consequence of providing measurement values of a variable in a plurality of, or disjoint, subregions of a parameter range may be that the measurement values of the variable are afflicted by measurement errors in the individual subregions, said measurement errors only occurring individually in the corresponding subregions. A varying measurement error may be present from subregion to subregion. Such a falsification of the measurement values of the variable has an adverse effect on the significance thereof.
In view of the above-described inspection system, establishing a wavefront aberration may serve to determine adjustment processes for adjusting the imaging optical unit in order to reduce the imaging aberration thereof. The use of a sensor at different measurement positions may be accompanied by incorrect positioning of the sensor. This may lead to measurement values of the wavefront aberration being respectively afflicted with a constant measurement error (offset) in individual subregions of the image field. The offset error could be significantly greater than other measurement errors and, in particular, greater than the field variation of the wavefront aberration caused by the imaging optical unit. This makes it more difficult to determine suitable adjustment processes. Such a disadvantage may correspondingly occur when using a sensor with a plurality of non-overlapping partial sensors. Here, offset errors occurring in subregions of the image field may be a consequence of positional errors of the partial sensors.